It is known that in saturated aqueous solutions certain salts maintain relative humidity at constant temperature. Excess salts in these solutions are required to cause the solution to remain saturated. American Society for Testing Materials (ASTM) Standard No. 104 describes the utility of this property of salt solutions in testing materials, components, devices, and parts for resistance to different humidity conditions. Electrical tests, for example, are conducted on printed circuit substrate materials from low to elevated temperatures and relative humidity to determine their stabiliy properties and the projected life at use conditions. These tests have widespread use in laboratories and could also have potential use in production environments to indicate how a particular material reacts to specific combinations of humidity and temperature.
The excess salt used to maintain a saturated solution causes a problem that heretofore has not been adequately addressed. The before-mentioned ASTM Standard No. 104 discusses the problem in this way:
"Creepage distance over the surface of the container between the solution and the material being conditioned should be long enough to prevent the solution from creeping to the material being conditioned. Creeping is more likely to occur with some of the salts than with either glycerin or sulfuric acid solutions."
Thus, even the ASTM standard acknowledges that the problem exists, but does not address the elimination of the problem. Rather, it suggests that the problem can be avoided by moving the part or material being tested away from the salt solution so as to prevent the salt creepage from reaching the part. Many times this approach is not feasible as the part or material being tested is in an enclosed test chamber which may not allow an adequate distance between the part or material being tested and the solution. This salt creepage or migration oftentimes will contaminate the part or material being tested, thus rendering the test invalid. Salt creepage can occur at room temperature, but it occurs most frequently and rapidly at elevated temperatures, especially at temperatures above 85 degrees C. Salt contamination is a particular problem in the case of electrical tests for printed circuit materials in which the salt migration can alter the failure mechanism and invalidate the test. Ideally, resistance to salt should not be a factor in the testing procedure.
It is known that a ring of silicone grease compound applied to a container above the solution can be used to prevent the salt migration. One problem with this method is that the grease cannot be allowed to come into contact with the aqueous solution, as the grease will alter the solution's composition and thus change the test result. Another problem is the wide variations in the material characteristics of different lots and brands of the grease. Furthermore, the excess salt from solution has a tendency to push through the silicone grease and thereby still contaminate the part.
It is further known that plating and etching solutions and the like contain excess salts for maintaining proper conditions in the solution. Typically these salts are poisonous and can cause serious physiological injury. With the excess salt in solution salt could migrate from the solution under the proper conditions, thereby creating a hazardous situation for the users of such solutions.
It is an object of this invention to prevent the excess salt in an aqueous solution from creeping or migrating up the sides of a container, especially in cases of elevated temperature and relative humidity, and thereby prevent the salt contamination of materials or parts being tested in enclosed environments.
It is another object of this invention to prevent the excess salt in an aqueous solution especially at elevated temperatures and relative humidity from creeping or migrating up the sides of a container whereby the composition of the solution will remain essentially unaffected.
It is yet another object of this invention to prevent the excess salt in aqueous solution especially at elevated temperatures and relative humidity from creeping or migrating up the sides of a container and allow dangerous or hazardous salts to escape from the container and cause physiological harm to the user.